1. Field of the Invention
The present invention generally relates to testing of grounding jumper cables and, more particularly, is concerned with a grounding jumper cable tester and testing method for determining if the resistance of a grounding jumper cable to current flow is below a predetermined safe level.
2. Description of the Prior Art
During maintenance of a power line, it may become accidentally energized due to the inadvertent closing of a breaker or switch or due to mutual induction from parallel live lines. To protect workers from electric shock should a fault occur, grounding jumper cables are installed in various configurations to form a jumper system. Grounding jumper cables are conductive cables that have attachment means, such as clamps, at either end. The jumper system provides a path for current to flow around the worker to a neutral line or to the earth. However, in order for the jumper system to adequately protect the worker, it must provide a very low resistance path to current flow so that the voltage drop across the worker is within a safe level should a fault occur.
The maximum allowable resistance of the jumper system can be determined based upon assumptions concerning the available fault current at the work site, the duration of the available fault current, and the level of safety to be provided. Once the maximum allowable jumper system resistance is determined, individual grounding jumper cables can be selected to form the jumper system. Since the resistance of the jumper system is determined primarily by the resistance of each of the individual jumper cables, it is necessary to determine the resistance of the individual jumper cables. Additionally, since they may be damaged during storage, transport or use, individual jumper cables should be periodically tested to ensure that their resistance remains suitable for the desired level of protection.
Grounding jumper cable tester devices exist, such as the one manufactured by Hasting's Fiberglass Corporation, that are designed to test the resistance of a jumper cable to current flow. The devices test the resistance of a jumper cable by relying on Ohm's law which defines the mathematical relationship between voltage, current and resistance. A known current is applied through the cable and the voltage drop across the cable is measured. With the current and voltage known, the resistance can be determined. However, the prior art jumper cable testing device identified above utilizes a current transformer to provide an alternating test current. This presents two significant drawbacks.
The first drawback is the inductance associated with alternating current. When alternating current flows through the jumper cable, inductance can function similar to resistance and cause a voltage drop. Such an induced voltage drop may result in an inaccurately high resistance determination. The effect of inductance is particularly acute if the cable is coiled or on a conductive surface. Thus, when using an alternating test current, it is critical to take steps to ensure that the jumper cable is positioned properly to minimize the possible effect of inductance.
The second drawback is the low testing voltage. Many jumper cables have aluminum clamps. A thin aluminum oxide coating forms on the clamps. This coating is highly resistant to current flow but easily breaks down when a sufficient voltage level, in the range of 5 to 10 volts, is applied. The voltage levels encountered at work sites are more than sufficient to break down any aluminum oxide coating on the clamps making the resistance of the coating irrelevant. However, if sufficient voltage is not applied during testing, the thin aluminum oxide coating can raise the resistance of the jumper cable and result in the unnecessary rejection of the jumper cable. While the current transformer used in the prior known devices provides a sufficiently high alternating test current, it does so at the expense of voltage. The voltage drop created by prior alternating current jumper cable test devices is often inadequate to break down the aluminum oxide coating resulting in an inaccurate resistance determination and unnecessary rejection of the jumper cable.
Consequently, a need still exists for a grounding jumper cable testing device and testing method that avoids the problem of inductance associated with alternating current and provides a sufficient voltage drop to break down any aluminum oxide coating on the clamps of the cable.